Such a method is known per se from U.S. Pat. Nos. 4,793,843 and 5,188,648 granted to the present applicant. From said the documents it is known that part of the dopant in the layers in the center may evaporate upon collapsing of the quartz substrate tube while heating. Said evaporation results in a disturbance of the refractive index profile in the final fibre. Said disturbance of the refractive index profile has an adverse effect on the bandwidth of the optical fibre.
The future developments in the telecommunication industry include the transmission of information at ever higher bit rates (bits/sec) over even longer distances. The present data networks use relatively low bit rates. Thus, light emitting diodes (LED) have so far been the most common light source in these applications. Because of the demand for data transmission rates which are higher than the modulation capacity of LEDs, laser sources will be used instead of LEDs. This shift manifests itself in the use of systems which are capable of supplying information at rates as defined in the GigabitEthernet Standard (IEEE 802.3z. 1998) and higher rates. GigabitEthernet Standard corresponds to a transmission rate of 1.25 Gigabit/sec.
The multimode optical fibre that is currently used in telecommunication systems has mainly been designed for being used with such LED light sources. In addition, said multimode fibre has not been optimized for use with the laser sources that are present in systems that have been designed for transmitting information at rates equal to or higher than GigabitEthernet. In other words, laser sources impose different demands on the quality and the design of a multimode fibre than LED sources. Especially, the refractive index profile in the center of the core of multimode fibres is of major importance, in which in particular a precisely defined parabolic profile is required so as to prevent a decrease of the information transmission rates. Accordingly, minor deviations in the center of the fibre profile may cause significant disturbances in the output signal, which disturbances have a major influence on the behavior of the system. This effect may manifest itself in the form of a very small bandwidth or a very high jitter, or both.
The wavelengths at which the data transmission in such fibres takes place are, respectively, the 850 m band, which is defined as 770 nm–920 nm herein, and the 1300 nm band, which is defined as 1260 nm–1360 nm herein.